The addictiveness of certain drugs and compulsive behaviors is linked to excitation of dopamine mediated reinforcement/reward pathways in the central nervous system (Abbott (2002) Nature 419:872-874; Montague et al. (2004) Nature 431:760-767). Normally dopamine functions to motivate mammals to perform behaviors important for survival, such as eating and sex, but in subjects with addictions, dopamine induces maladaptive behavior. Subjects with addictions feel compelled to use a substance or perform a behavior repeatedly despite experiencing harmful effects. Virtually all drugs of abuse and compulsive behaviors have been shown to increase extracellular dopamine concentrations in the nucleus accumbens of mammals.
Drugs of abuse induce dopamine-mediated dependence characterized by compulsive drug craving and drug seeking behaviors. The World Health Organization (WHO) has classified addictive drugs into nine groups: 1. alcohol, 2. amphetamines, 3. barbiturates, 4. marijuana, 5. cocaine, 6. hallucinogens, 7. khat, 8. opiates, and 9. organic solvents. Dysregulation of dopamine pathways is also associated with compulsive behavioral addictions, such as excessive eating, drinking, smoking, shopping, gambling, sex, and computer use (Comings et al. (2000) Prog. Brain Res. 126:325-341; Comings et al. (1997) 2:44-56; Blum et al. (2000) J. Psychoactive Drugs 32 suppl:i-iv, 1-112; Potenza (2001) Semin. Clin. Neuropsychiatry 6:217-226; Gianoulakis (1998) Alcohol Health Res. World 22:202-210; Bowirrat et al. (2005) Am. J. Med. Genet. B Neuropsychiatr. Genet. 132:29-37;  Di Chiara (2005) Physiol. Behav. 86:9-10; Franken et al. (2005) Appetite 45:198-201; Wang et al. (2004) J. Addict Dis. 23:39-53; Aamodt (1998) Nature Med. 4:660; and Koepp et al. (1998) Nature 393:266-268).
In addition, physical and psychological dependence accompanied by withdrawal syndrome is often associated with use of addictive drugs and compulsive behavior. Withdrawal is defined as the appearance of physical and behavioral symptoms upon reduction or cessation of drug use or compulsive behavior. Withdrawal reflects changes occurring in the central nervous system in response to continued use of a substance or repetition of addictive behavior that usurp the normal mechanisms mediating reinforcement and reward of behavior to motivate the addicted individual to continue consuming a drug or repeating compulsive behavior in the face of serious social, legal, physical and professional consequences. Physical symptoms of withdrawal may include intense cravings, irritability, anxiety, dysphoria, restlessness, lack of concentration, lightheadedness, insomnia, tremor, increased hunger and weight gain, yawning, perspiration, lacrimation, rhinorrhoea, dilated pupils, aching of bones, back and muscles, piloerection, hot and cold flashes, nausea, vomiting, diarrhea, weight loss, fever, and increased blood pressure, pulse and respiratory rate.
The management of opioid withdrawal syndrome has long been recognized as an unmet clinical need. Chronic pain afflicts upwards of one in three adults worldwide. Opioid compounds, such as morphine, are frontline therapeutics for the control of chronic pain. Because chronic pain, by definition, persists for many months (and up to the remainder of the patient's life), morphine and like compounds may be given chronically as well. This is a dire problem because opioids induce dependence upon repeated administration, meaning that continuing administration of opioids is required for patients to function normally. When opioids are discontinued, and also during the temporal lag between successive doses of opioids, the patient goes into withdrawal.
Because opioids exert actions in a wide array of brain, spinal cord and bodily tissues, the effects of opioids, and consequent withdrawal symptomologies, are diverse. The signs of withdrawal are generally opposite to the effects of opioids. For example, morphine causes constipation; withdrawal causes diarrhea. Morphine decreases core body temperature, withdrawal raises it. Morphine causes sedation, withdrawal causes agitation. Additional signs of withdrawal include increased pain, dilated pupils, goose pimples, yawning, cramps, muscle aches, restlessness, extreme anxiety, insomnia, nausea and vomiting, sweating, tearing, tachycardia, and increased blood pressure.
Perversely, although pain reduction is the reason that opioids are administered, pain dramatically rebounds during withdrawal such that pain is not only not controlled by the opioids in the area of the original pain complaint, but rather the entire body is now extraordinarily sensitive to touch and temperature stimuli, misinterpreting ordinarily nonpainful stimuli as painful. Light touch becomes painful. Warm and cool become painful. This twist of everyday sensation into threatening pain (along with the other withdrawal symptomology) destroys, on a daily basis, the lives of many millions in the U.S. alone. It creates great suffering in chronic opioid recipients, in patients needing to discontinue opioids, and in recovering drug addicts, whose desire to avoid withdrawal symptoms may prevent them from escaping from illicit drug use.
The problem is compounded by the fact that there is currently no remedy for withdrawal, short of another dose of opioid. As addicts know, another dose of the drug does nothing to solve the problem but instead only masks the problem until the drug yet again wears off. Current approaches to bringing patients and addicts through withdrawal are dire, including “cold turkey”, sedation, and analgesia. “Detoxification” is often induced with naltrexone (an opioid receptor antagonist) under general anaesthesia or benzodiazepine sedation, in a closely monitored environment such as intensive care. Naltrexone induces acute withdrawal, with symptoms that last for about six days. It is only considered for patients in good health. Other currently employed methods to take humans through withdrawal include administration of non-steroidal anti-inflammatory drugs such as paracetamol, anti-emetics such as metoclopramide, anti-diarrheals such as loperamide, diazepam to reduce anxiety and agitation, and clonidine to decrease anxiety, sweating, and changes in heart rate and blood pressure.
In developing an improved treatment for opioid withdrawal it is important to consider that opioids, including morphine, do not just affect neurons. While opioid-responsive neurons in various brain and spinal cord regions suppress pain, lower core body temperature, alter hormone release, etc. (the classical effects of opioids), it has recently been discovered that opioids also affect a non-neuronal cell type called glia (microglia, astrocytes, oligodendrocytes). Morphine and other opioids activate glia. This activation increases with repeated opioid administration, as evidenced by the upregulation of glia-specific activation markers. That such glial activation contributes to morphine tolerance is supported by the finding that co-administering glial inhibitors along with morphine disrupts the development of morphine tolerance. It follows that reduction of glial activation may be useful as a therapeutic approach to disrupting the development of morphine tolerance. Watkins, L. R. et al. (2005) Trends in Neuroscience 28:661-669; Gul, H. et al. (2000) Pain 89:39-45;  Johnston, I. N. et al. (2004) J Neurosci. 24:7353-65; Raghavendra, V. et al. (2002) J Neurosci 22 (22):9980-89; Raghavendra, V. et al. (2004) Neuropsychopharmacology 29 (2):327-34; Shavit, Y. et al. (2005) Pain 115:50-59; Song, P. and Zhao, Z. Q. (2001) Neurosci. Res. 39:281-86.
Opioid-driven progressive glial activation causes glia to release neuroexcitatory substances, including the proinflammatory cytokines interleukin-1 (IL-1), tumor necrosis factor (TNF), and interleukin-6 (IL-6). These neuroexcitatory substances counteract the pain-relieving actions of opioids, such as morphine, and drive withdrawal symptomology, as demonstrated by experiments involving co-administration or pro- or anti-inflammatory substances along with morphine. For example, injecting IL-1 into the cerebrospinal fluid of mice at a dose having no behavioral effect on its own blocks the analgesic effect of systemic morphine. Similarly, spinal delivery of morphine and IL-1 receptor antagonist (which prevents IL-1 from exerting its effects), or morphine and the anti-inflammatory cytokine IL-10 (which downregulates the production, release and efficacy of proinflammatory cytokines), enhances the magnitude and duration of morphine analgesia. Indeed, if morphine analgesia is established and then allowed to dissipate, potent analgesia can be rapidly reinstated by injecting IL-1 receptor antagonist, suggesting that dissipation of analgesia is caused by the activities of pain-enhancing proinflammatory cytokines rather than dissipation of morphine's analgesic effects.
The activity of other opioids may also be opposed by activation of glia. Studies show that glia and proinflammatory cytokines compromise the analgesic effects of methadone, at least in part, via non-classical opioid receptors (Watkins, L. R. et al. (2005) Trends Neurosci. 28:661-669). These results suggest that glia and proinflammatory cytokines will be involved in methadone withdrawal, and likely withdrawal from other opioids as well. These data also expand the clinical implications of glial activation, since cross-tolerance between opioids may be explained by the activation of the glial pain facilitatory system, which undermines all attempts to treat chronic pain with opioids.
In summary, opioids excite glia, which in turn release neuroexcitatory substances (such as proinflammatory cytokines) that oppose the effects of opioids and create withdrawal symptoms upon cessation of opioid treatment. Compounds that suppress such glial activation would be beneficial novel therapeutics for treatment of opioid withdrawal.
There remains a need for improved compounds, compositions, and methods of treatment for drug and behavioral addictions. In particular, drugs are needed that attenuate or abolish the dopamine mediated “reward” associated with addicts' cravings and that alleviate symptoms of withdrawal syndromes after discontinuance of drug use or compulsive behavior.